Homeless persons with alcohol and other drug (AOD) disorders face multiple problems that go beyond their AOD use including extreme poverty, unmet subsistence and housing needs, poor social support, comorbid mental illness, poor physical health, and elevated rates of criminal justice system involvement. They thus commonly interact with multiple public sector service agencies in addition to the AOD treatment system. Because these public sector service agencies operate independently of one another, it has been very difficult to obtain an understanding of what this patchwork "system" of care for homeless adults with AOD disorders looks like, how much it costs, the way in which costs are distributed across different agencies, and the extent to which care is coordinated between system sectors to ensure that services are being delivered in an efficient manner. This study will begin filling this gap by providing the first estimates of the extent to which homeless persons with AOD disorders use AOD, mental health, criminal justice, social welfare, and medical services, the costs associated with such use, and the factors that predict use and cost. It will also identify barriers to developing a more coordinated and broader cross-system response to managing the care of homeless adults with AOD disorders. To address these aims, the study will draw upon a unique data set created for an NIMH-funded study, "Public Sector Costs of the Homeless Mentally III". This data set links, at the client-level, (a) survey data collected through face- to-face interviews with a probability sample of 800 homeless adults in Houston and (b) a year of administrative data on their service use and associated costs that were obtained from 11 public sector service systems that together comprise the health, mental health, social service and criminal justice systems of care. These data will be supplemented with organizational case studies of how these systems are interacting with reference to homeless adults with AOD disorders that will allow us to better interpret our analyses of existing data and make more pointed policy recommendations regarding how to improve the coordination of services for this population.